Stephane, it's not deep. People who need to write doctests that work across N versions of Python shouldn't need to read N versions of the documentation. This is hardly unique to doctest. We routinely add "Changed in version m.n" blurbs all over the place.
Ways that were _necessary_ for robust dict testing continue to work fine in 3.6 and 3.7, so it doesn't harm anything if people mindlessly copy an example that _could_ be spelled some other way under 3.6+. It's not like there's even anything slightly obscure about, e.g.,
>>> foo() == {"Hermione": "hippogryph", "Harry": "broomstick"}
True